In general, various sensors are installed in a washing machine to sense several washing conditions necessary for a washing operation. The washing machine is appropriately driven according to the sensed washing conditions in order to obtain the maximum washing effect. As sensors of a conventional washing machine, a sensor for sensing an amount of washing water received in a washing tub and a sensor for measuring a water temperature in order to boil the laundry have been proposed and used.
Meanwhile, one of factors that determine properties of the washing water is a water hardness. The water hardness is a term that represents a degree of a water strength. Materials determining the hardness are calcium, magnesium, iron, strontium, and the like. As well known, in case the water harness is high, a detergent is not dissolved well due to magnesium ions or calcium ions contained in the water, so that a cleaning capacity of the detergent is degraded. Accordingly, the water having a high hardness is not suitable for domestic water or industrial water.
However, the related art has not proposed a method for adjusting an amount of detergent and water in the washing machine according to the hardness of the washing water. Although different areas have different water hardness, the equal method of inputting detergent and water is applied at every area, which is one of factors that degrade the washing efficiency. Specifically, in an area where underground water is used as the washing water, an equal amount of detergent and water is used while the hardness of the washing water is ignored. This is an important factor that degrades the washing efficiency.